The point being that the answer you get will depend a lot on the exact question you ask. Is it the number of people working in the field? The number of articles in Phys. Rev. Lett.? How much money [big name university] is willing to put to recruit a leader in the field?
If you want to get an idea of what physicists themselves think is exciting, I suggest you look at the research highlighted at
If we go purely by number of people working on it then every field is hotter than each of its subfields.
"Nuclear and particle physics" might be hotter than laser physics, but laser physics could be hotter than both nuclear physics and particle physics. Until you split laser physics into solid-state lasers and other lasers, then you get the reverse order again. Made-up example, but it should demonstrate the problem.
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